"Empty cognizance of one taste, suffused with knowing, is your unmistaken nature, the uncontrived original state. when not altering what is, allow it to be as it is, and the awakened state is right now spontaneously present."

Ok before I answer, I dont think thats how the power gem works. I'm not 100% sure, I just copy pasted the description from Marvel wiki.

Anyway, I am not sure who is more powerful. Both of them are super powerful and still have no idea how each of them lost. Thats why I am reading the Onslaught saga comics so I can learn the truth. But they both kinda look like they can beat each other, its a matter of circumstances. Onslaught could invade his mind and they both could have a mind battle, i kinda think onslaught would win. Thanos can use the reality gem and erase him from reality, if the gauntlet is made of metal onslaught could take it away by using magneto's powers. Thanos might get arrogant which could give Onslaught an opening, its a matter of circumstances.

"Empty cognizance of one taste, suffused with knowing, is your unmistaken nature, the uncontrived original state. when not altering what is, allow it to be as it is, and the awakened state is right now spontaneously present."